The Czochralski process in which a large monocrystalline boule is drawn slowly from a melt of solidifying semiconductor, starting from a single seed crystal, is of major importance in the production of silicon and other semiconductor devices. In certain respects, the quality of the crystalline structure produced by this process exceeds that producible by the other major competitive process, zone refining.
In order to achieve the excellent quality of crystals which are needed by the semiconductor industry, a number of process variables in the Czochralski process must be carefully monitored and controlled. In particular, the dopant concentration in the melted semiconductor from which the crystal is being pulled must be carefully adjusted to preserve constancy during the process of growing the crystal. This is true especially because the concentration of dopant in the finished crystal is different from that in the molten bath of semiconductor. Consequently, during the process of growing a crystal, there is a tendency for the dopant to either be depleted or overly concentrated because of the differential in segregation coefficients of semiconductor and dopant.
There is furthermore a tendency for the diameter of the crystal to change during the process of drawing it. Since the boule or monocrystal must be ground to a uniform diameter after drawing, any variations in the as-drawn diameter simply require extra grinding, increasing cost and the risk of damage to the semiconductor crystal structure. To minimize such diameter variations the rate at which the solidifying crystal is drawn must be continuously optimized.
Consequently, these and other process parameters have come to be quite rigidly controlled in order to assure maximum quality of the finished semiconductor wafers which will be sliced from the boule. Such control has proven to be feasible more or less at the expense of production speed, by drawing only a single crystal from each "melt" of silicon, for example. At the termination of the crystal drawing process, the furnace is shut down, a new charge of solid silicon is added, the furnace is heated to melt the silicon and the entire process of drawing a new crystal is begun.
While these sorts of batch process have proven adequate to provide silicon wafers at a satisfactorily low cost for semiconductor device fabrication, for example, the resulting cost of the silicon wafers is still too high to permit their widespread application as photovoltaic converters of sunlight.
In order to meet the need for semiconductor wafers which are sufficiently inexpensive, high enough in quality, and abundantly available, it is apparent that the former batch processing technique used in the Czochralski process must be abandoned in favor of a more or less continuous process in which the drawing crucible is recharged with molten semiconductor during the crystal drawing process so that more than one crystal can be drawn without a shutdown to recharge the furnace. Moreover, the already well established production controls over the various process parameters must be extended to this new continuous process such that the result is a net lowering of the cost of the semiconductor boule without any degradation in quality. In short, the yield of acceptable boules (the percentage of those produced which meets quality standards) must be preserved while efficiency and production speed are increased.
Consequently, a means is definitely needed to provide in a simple, continuous, and reliable fashion, a constant melt level in the drawing crucible such that the position of the solid-liquid interface in the crucible does not change. Moreover, this criterion must be met despite the fact that the crucible remains continuously heated, and crystals are formed one after another in a production fashion.
At the same time, the composition of the molten semiconductor must remain uniform. In particular, the concentration of dopant must not be permitted to change either during the growing of a single crystal or from crystal to crystal. Moreover, this last criterion must be met even though the total amount of molten semiconductor present in the drawing crucible at the start of production cannot be made adequate to provide material for an entire production run which might consist of many crystals.
Considering the above criteria, it is obvious that if quality is to be maintained in a mass production crystal growing process, then replenishment of the molten semiconductor in the drawing crucible must be carried out during the production run, with careful control of the composition of the added molten semiconductor.